# Function Service Implementation

## Overview

This document describes the implementation of the Function Service for Spearlet, which provides a comprehensive gRPC-based API for function execution, task management, and monitoring capabilities.

## Architecture

### Proto Definition (`function.proto`)

The Function Service is defined in `proto/spearlet/function.proto` and follows the same architectural patterns as the existing Object Service:

#### Core RPC Methods

1. **Function Execution**
   - `InvokeFunction`: Execute functions with sync/async modes
   - `GetExecutionStatus`: Query execution status and results
   - `CancelExecution`: Cancel running executions
   - `StreamFunction`: Stream-based function execution

2. **Task Management**
   - `ListTasks`: List available tasks with pagination
   - `GetTask`: Retrieve specific task information
   - `DeleteTask`: Remove tasks from the system
   - `ListExecutions`: List execution history

3. **Health & Monitoring**
   - `GetHealth`: Service health status with detailed metrics
   - `GetStats`: Comprehensive service statistics

#### Message Structures

**Request/Response Patterns:**
- All requests follow consistent naming: `{Method}Request`
- All responses follow consistent naming: `{Method}Response`
- Pagination support with `limit`, `start_after`, and `has_more` fields
- Optional fields for detailed information (`include_details`, `include_logs`)

**Key Data Structures:**
- `TaskInfo`: Task metadata and configuration
- `ExecutionInfo`: Execution status and results
- `HealthDetails`: Service health metrics
- `ServiceStats`, `TaskStats`, `ExecutionStats`: Statistical information

### Rust Implementation (`function_service.rs`)

The Rust implementation follows the established patterns from `object_service.rs`:

#### Core Components

1. **FunctionServiceImpl**
   ```rust
   pub struct FunctionServiceImpl {
       kv_store: Arc<dyn KvStore>,
       stats: Arc<RwLock<FunctionServiceStats>>,
       default_timeout_ms: u64,
       max_concurrent_executions: usize,
       start_time: std::time::Instant,
   }
   ```

2. **Statistics Tracking**
   ```rust
   pub struct FunctionServiceStats {
       pub total_tasks: u64,
       pub total_executions: u64,
       pub running_executions: u64,
       pub successful_executions: u64,
       pub failed_executions: u64,
   }
   ```

3. **Storage Models**
   - `StoredTask`: Serializable task information
   - `StoredExecution`: Serializable execution data

#### Implementation Patterns

**Consistent Error Handling:**
- All methods return `Result<Response<T>, Status>`
- Proper gRPC status codes for different error conditions
- Detailed error messages for debugging

**KV Store Integration:**
- Tasks stored with prefix `task:`
- Executions stored with prefix `execution:`
- Atomic operations for consistency

**Statistics Management:**
- Thread-safe statistics using `Arc<RwLock<T>>`
- Real-time updates during operations
- Comprehensive metrics collection

## Integration with gRPC Server

### Server Registration

The Function Service is integrated into the main gRPC server (`grpc_server.rs`):

```rust
pub struct GrpcServer {
    object_service: Arc<ObjectServiceImpl>,
    function_service: Arc<FunctionServiceImpl>,  // Added
    health_service: Arc<HealthService>,
}
```

### Health Service Integration

The health service now includes function service metrics:

```rust
pub struct HealthStatus {
    pub status: String,
    pub uptime_seconds: i64,
    pub object_count: u64,
    pub total_object_size: u64,
    pub task_count: u64,        // Added
    pub execution_count: u64,   // Added
    pub running_executions: u64, // Added
}
```

## Key Design Decisions

### 1. Consistency with Object Service
- Same architectural patterns and code structure
- Consistent naming conventions and error handling
- Similar KV store usage patterns

### 2. Comprehensive API Surface
- Full CRUD operations for tasks
- Detailed execution tracking and monitoring
- Rich statistics and health information

### 3. Scalability Considerations
- Configurable timeout and concurrency limits
- Efficient pagination for large datasets
- Atomic operations for data consistency

### 4. Monitoring and Observability
- Detailed health checks with service-specific metrics
- Comprehensive statistics collection
- Real-time status tracking

## Future Enhancements

### Proto Generation
- The proto files need to be regenerated to create the gRPC server stubs
- Integration tests will be added once proto types are available

### Implementation Details
- Actual function execution logic (currently placeholder)
- Advanced task scheduling and queuing
- Distributed execution capabilities
- Enhanced monitoring and alerting

## Testing Strategy

### Unit Tests
- Basic service creation and initialization
- Statistics tracking and updates
- ID generation utilities

### Integration Tests (Pending)
- Full gRPC method testing
- End-to-end execution workflows
- Error handling scenarios
- Performance and load testing

## Files Modified/Created

1. **Proto Definition**: `proto/spearlet/function.proto`
2. **Service Implementation**: `src/spearlet/function_service.rs`
3. **Test Framework**: `src/spearlet/function_service_test.rs`
4. **Module Integration**: `src/spearlet/mod.rs`
5. **Server Integration**: `src/spearlet/grpc_server.rs`

## Next Steps

1. Regenerate proto files to create gRPC stubs
2. Complete integration tests
3. Implement actual function execution logic
4. Add advanced monitoring and metrics
5. Performance optimization and load testing